injectables context manager

The sim.injectables context manager allows you to temporarily set
some injectables to given values for a limited scope.
Injectables should be returned to their original values when the
context manager exits, but changes to things like tables
could have lasting effects.
Function for checking whether a name is an expression
eval_variable function

This can be useful when you want to evaluate a single injectable,
table, or column and get back the result as it would be injected into
a function under the simulation framework.
eval_model function

Useful for evaluating a single model and gettings its return value.
Models often have side effects instead of return values,
and the simulation framework ignores model return values,
but this could be useful for model evelopment.
cache_disabled context manager

Disable cacheing for a specific block of commands.
Add some toolz laziness to labels dict creation
Docs about eval_variable and eval_model

[ci-skip]
Merge pull request #128 from synthicity/sim-exec

Evaluate variables and models a la carte
Update LICENSE
Duck type pandas DataFrame and Series in sim framework

Rather than using isinstance to check if an object is a
DataFrame or Series, support objects that act like them by
checking for presence/absence of attributes.
Ability to disable table copies in sim framework

By default, simulation framework always returns copies of evaluated
tables and their columns, instead of views. Add parameter and
attribute to disable copies where possible. This includes
computed columns but not pandas slices which are always copies.
Added test for copying when enabled and disabled.
Drop DataFrame and Series check in sim framework

Don't check for presence/absence of attributes, just assume that
the passed object is a DataFrame or Series if it's not a callable.

Thanks to @jiffyclub for the suggestion.
Python 3 support in simulation framework
Downgrade some dependency revision numbers

This allows use of system Python packages on Ubuntu 14.04 for all
dependencies, except toolz, which is not packaged. If we need the
higher versions, we can revert this commit.